1. Field of the Invention
The present invention relates to an electromechanical transducer, such as a capacitive ultrasonic transducer, and a method for fabricating the same.
2. Description of the Related Art
In recent years, a capacitive electromechanical transducer fabricated using a micromachining process has been researched. A normal capacitive electromechanical transducer has a vibrating membrane supported at a distance from a lower electrode, and an upper electrode disposed on the surface of the vibrating membrane. This is used, for example, as a capacitive micromachined ultrasonic transducer (CMUT). A CMUT transmits or receives ultrasound using a light-weight membrane. It is easy to obtain a CMUT that has excellent broadband characteristics in both liquid and air. Use of this CMUT enables more accurate diagnosis than conventional medical diagnosis. Therefore, it is attracting attention as a promising technology. The operating principle of a CMUT will be described. When transmitting ultrasound, a DC voltage overlapped with an AC voltage is applied between the lower electrode and the upper electrode. This causes the vibrating membrane to vibrate, thereby generating ultrasound. When receiving ultrasound, the vibrating membrane is deformed by ultrasound, and the deformation causes a change in the capacitance between the lower electrode and the upper electrode. Therefore, the displacement of the vibrating membrane can be detected as an electric signal.
A method called “surface type” is used as a method for forming the “gap” (the distance between the electrodes) of a CMUT. It includes providing a sacrifice layer having a thickness equal to the distance between the electrodes, forming a vibrating membrane on the sacrifice layer, removing the sacrifice layer by etching, and sealing the etching hole. It is proposed to seal the etching hole with a SiN film formed by plasma-enhanced (PE) CVD or a SiO2 film formed by low-pressure (LP) CVD in view of the size of the etching hole (see IEEE Transactions On Ultrasonic, Ferroelectrics, And Frequency Control, Vol. 52, No. 12, December 2005, pp. 2242-2258). It is also proposed to seal the etching hole with metal (see U.S. Pat. No. 4,262,399).
The above-described surface-type CMUT has many sealing portions on the surface of the substrate, and therefore a sealing method is required by which the many sealing portions can be sealed collectively and tightly. Although a thin film can be formed by vacuum film formation, it is difficult to completely seal the vertical hole structure of the etching hole in the surface of the vibrating membrane with a deposited film formed by gas phase reaction in a vacuum atmosphere, for example, by CVD or PVD. When the etching hole in the vibrating membrane is sealed by CVD or the like, a deposited film is formed on the side wall defining the hole or a part of the vibrating membrane around the etching hole, and this film grows toward the center of the hole (deposition progresses in the lateral direction). However, in general, in gas phase reaction, active species (ions, radicals, or the like) that form a deposited film tend to travel in a straight line, and the deposition rate in the lateral direction is low. Therefore, it is necessary to increase the film thickness by prolonged film formation. If there is a minute sealing defect in the wet step after the sealing step, the solution used for developing and removing photoresist may enter the cavity and cause a malfunction. On the other hand, when a resin material that has fluidity is used for sealing, the vertical hole structure can be tightly sealed due to fluidity. However, in the process of hardening, the solvent in the material that has fluidity may contaminate the cavity by being heated, or the material may become temporarily less viscous due to high temperature and may flow into parts that need not be sealed, and the function of the device may thereby be interfered with. The present technical situation is as described above. For example, in medical diagnosis, CMUTs are normally used in contact with liquid such as castor oil in order to prevent attenuation of ultrasound that is a signal. If the reliability of the sealing portion is low, oil may enter the cavity in prolonged use, and the performance degradation may be caused.